Peter Frost's anthropology blog, with special reference to sexual selection and the evolution of skin, hair, and eye pigmentation

Saturday, September 10, 2011

Did the slave trade make Africans polygynous?

Slave exports to the Americas from different parts of Africa (Dalton & Leung, 2011). Did the slave trade create patterns of behavior that today exist throughout sub-Saharan Africa, such as generalized polygyny?

Why is polygyny so frequent in sub-Saharan Africa? As Goody (1973, p. 177) noted, the differences with Eurasia are striking:

In Europe and Asia, polygyny is largely but not exclusively an heir-producing device; often it is a way of replacing a barren wife. In Africa, plural marriage is far more generalised; according to Dorjahn, about 35 per cent of married men have more than one wife. Hence a large percentage of the population is likely to be part of a polygynous unit at some point in the life-cycle. Most men will be polygynously married at some time or other; women are yet more likely to be so. And most siblings will have sets of half siblings, both because of the plural marriage of their fathers and because of the remarriage of their mothers — since polygyny inevitably involves a large differential in the age of marriage, men will be older when they beget children than women are when they bear them. Hence there will be a higher proportion of widows and fatherless children.

Goody (1973) attributes this generalized polygyny partly to female self-reliance in food production. Year-round farming enables women to provide for their own needs and those of their children. A wife thus costs little in terms of upkeep, and this low maintenance cost encourages men to have as many wives as possible.

This rule nonetheless has interesting exceptions. In the savannah regions of Ghana, women plant grain and help with the harvest, but they leave yam cultivation to men and do not engage in hoeing for cereal agriculture. Yet polygyny rates are somewhat higher there than elsewhere in Ghana, where women contribute more to food production. Polygyny is also less frequent in East Africa than in West Africa, yet women contribute more to food production in East Africa than in West Africa.

Goody (1973, p. 185) concludes that “hoe agriculture, female farming and polygyny are clearly associated in a general way” but there must be other explanatory factors. But what?

For Dalton and Leung (2011), one big factor is the slave trade—the mass exportation of African laborers that ended only two centuries ago. West Africa tended to export male slaves while East Africa tended to export female slaves. This pattern reflected differences in market demand: on the one hand, the Americas wanted farm labor; on the other, the Middle East and South Asia wanted domestics or concubines. These differing sex ratios might therefore explain why polygyny is less frequent in East Africa than in West Africa:

The slave trades existed for hundreds of years, and, as a result, Africa experienced abnormal sex ratios for long periods of time. Polygyny could have emerged or been strengthened during the long period of abnormal sex ratios. Figures 1 and 2 suggest the Western Coast should have contained more polygynous marriages, whereas the Eastern Coast should have contained fewer.

These abnormal sex ratios returned to normal once the slave trade had ended in the early 19th century. Why, then, didn’t polygyny rates follow this return to normal? Dalton and Leung (2011, p. 8) blame cultural conservatism: “Once these cultural traits are established, polygyny can become self-sustaining.”

This hypothesis is interesting and might explain some of the variation in polygyny rates within sub-Saharan Africa. But it fails to explain why polygyny rates are in the double-digit range throughout sub-Saharan Africa. East Africa’s rates are lower but still high by world standards. Goody (1973, p. 181) states that East African cattle societies have a rate of 24.7%, i.e., the percentage of married men with more than one wife. Pebley and Mbugua (1989) similarly write: “The frequency is somewhat lower in East and South Africa, although 15 to 30 percent of husbands are reported to be polygynists in Kenya and Tanzania.”

[…] the percentage of men in polygynous marriages in Western African countries like Guinea, Togo, and Benin is 35.037, 29.793, and 29.679, whereas in Eastern African countries like Ethiopia, Kenya, and Malawi the percentage is 6.131, 9.206, and 10.101.

These figures come from the latest “Demographic and Health Surveys” and are twenty to forty years more recent than the other figures. As such, they reflect the decline in female farming and the growing urbanization of African societies. In this new context, institutionalized polygyny has given way to looser arrangements with multiple girlfriends and/or prostitutes. And why bring Ethiopia into the comparison? It is not a sub-Saharan country and differs from both West and East Africa in many ways, notably the long-established influence of the Coptic Church and Christian sexual norms.

Dalton and Leung also err in assuming that sub-Saharan Africa had low polygyny rates before the slave trade. Several lines of evidence argue otherwise:

1. The ratio of Y-chromosome to X-chromosome variability is much higher among sub-Saharan Africans, New Guineans, and Aboriginal Australians than among other human populations. This suggests a long-lasting trend of fewer men than women contributing to the gene pool (Dupanloup et al., 2003; see also Torroni et al., 1990; Scozzari et al., 1997).

2. Proto-Bantu, spoken approximately 3,000 years ago, has a specific term for “taking a second wife” (Polome, 1977).

3. A high level of male-male competition for females is suggested by the increased sexual dimorphism of African Americans for weight, chest size, arm girth, and leg girth (Todd & Lindala, 1928; Wolff & Steggerda, 1943). In contrast, a small, gracile, and almost childlike body characterizes Khoisans and Pygmies, the only sub-Saharan populations with low polygyny rates.

After arguing that the slave trade caused generalized polygyny, Dalton and Leung see therein a leading cause of Africa’s lag in economic and social development.

There is undoubtedly some kind of relationship between generalized polygyny and Africa’s stubbornly high fertility and economic poverty, but it’s not a simple one of cause and effect. Banning polygyny will not cause immediate changes to reproductive and economic behavior. Dalton and Leung themselves argue that cultural conservatism alone has maintained high polygyny rates in Africa for the past two centuries. Wouldn’t the same be true for other behaviors?

Remember also that the time depth of generalized polygyny is not the four or five centuries that Dalton and Leung claim. In sub-Saharan Africa, high polygyny rates are associated with ‘female farming’ societies, and such societies began to spread outward from a point of origin near the Niger’s headwaters some 6,000 to 7,000 years ago (Murdock, 1959, pp. 44, 64-68).

Behavioral predispositions have significant heritability, especially in relation to sexual behavior (Comings et al., 2002; Mendle et al., 2006; Belsky et al., 2007). If generalized polygyny has existed in sub-Saharan Africa for six to seven thousand years, wouldn’t it have favored certain predispositions and not others? And wouldn’t those predispositions survive the banning of polygyny?

The question doesn’t seem to have crossed the authors’ minds. They seem to believe, a bit naïvely, that there is only cultural conservatism to worry about. No less naïve are the authors they cite:

Nunn and Wantchekon (2010) examines a particular channel through which the slave trades impact current African economic performances, namely the levels of trust across individuals within Africa. Trust supports economic exchange in well-functioning markets and would have plausibly been affected within groups living in the capture and export economies participating in the slave trades. Nunn and Wantchekon (2010) shows those individuals whose ancestral groups experienced higher slave exports exhibit lower levels of trust even to this day. Our paper contributes to these findings by suggesting an additional channel through which the slave trades have had a long-term impact on current African society. (Dalton & Leung, 2011, p. 2)

Low trust is typical of all simple, clan-based societies. Papua-New-Guinea was not affected by the slave trade, yet it has very low levels of trust. The slave trade may indeed have made people less trusting in some parts of sub-Saharan Africa than in others. But this does not explain why levels of trust are so low in sub-Saharan Africa as a whole.

And questions can be raised about the study by Nunn and Wantchekon (2010). Those authors found that willingness to trust members of other ethnic groups correlated with an ethnic group’s historical importance as a source of slaves. Most slaves, however, were taken during inter-tribal wars, and such wars generally occurred in regions already prone to interethnic conflict.

In short, the correlation is valid, but it doesn’t prove the causal relationship that Nunn and Wantchekon infer. In fact, the line of causality probably runs in the opposite direction.

Comings, D.E., D. Muhleman, J.P. Johnson, J.P. MacMurray. (2002). Parent-daughter transmission of the androgen receptor gene as an explanation of the effect of father absence on age of menarche. Child Dev., 73, 1046-1051.

from Peter:And why bring Ethiopia into the comparison? It is not a sub-Saharan country and differs from both West and East Africa in many ways, notably the long-established influence of the Coptic Church and Christian sexual norms.

The coastal tribes did the slaving so there would be a different dynamic between the coast and inland. The slaving tribes would have an excess of females while the target tribes would have a deficit.

Islam is part of it i think. The slave trade was originally Arabs coming from the east and Madagascar and north Africans coming from the west after sailing round the west coast. The coastal tribes became muslim through that contact. The wealthier men took multiple wives encouraging slave-raiding among the wifeless men.

The polygynous man in question comes from a predominantly Muslim region of Ethiopia.

Anon,

Many historians, notably Bernard Lewis, equate the rise of black slavery with the rise of Islam from the 7th century onward.

I think this is a myth. There is good evidence that the outflow of slaves from sub-Saharan Africa began to increase not long before the time of Christ. This increase was gradual and was probably driven by factors internal to sub-Saharan Africa (i.e., the expansion of 'female farming' societies, high polygyny, and endemic warfare). By late Antiquity, most slaves in the eastern Mediterranean appear to have been of African origin.

This long-term trend continued under Islam, but I don't think Islam in itself had anything to do with it.

I think this is a myth. There is good evidence that the outflow of slaves from sub-Saharan Africa began to increase not long before the time of Christ. This increase was gradual and was probably driven by factors internal to sub-Saharan Africa (i.e., the expansion of 'female farming' societies, high polygyny, and endemic warfare). By late Antiquity, most slaves in the eastern Mediterranean appear to have been of African origin.

You run the risk of being excoriated by the feminists.

You have just implied that female independence in provisioning their offspring leads to slavery!

What about disease being responsible for removing men from the societies of West Africa ? Men are more vulnerable to many diseases acording to J. Manning and the coastal regions of West Africa were the unheathiest places in the world. And if melanin protects against infections as Manning suggests the unheathiness of W. Africa could also explain why West Africans are so much darker than East Africans.

Re. "A high level of male-male competition for females is suggested by the increased sexual dimorphism of African Americans".

Yes, but the slaveholders bred African Americans for size and strength by using selected African males to impregnate their female slaves, black and white. (see 'To hell or Barbados: the ethnic cleansing of Ireland', Sean O'Callaghan - 2000)

given female self-sufficiency in farming in Africa (sub-Saharan?) an excess of males is problematic, because it leads to warfare.

It would seem to me that there could be selection on females in those environments to have more female offspring, since male offspring are a waste of time unless they are in very good condition, but in any case are nowhere as good an investment as female offspring are from a reproductive success point of view.

It would seem to me that there could be selection on females in those environments to have more female offspring, since male offspring are a waste of time unless they are in very good condition, but in any case are nowhere as good an investment as female offspring are from a reproductive success point of view.

We could probably quantify that, however, it would seem that females without access to lots of resources would be better off to have mostly daughters, while females who were wives of very powerful males would be better off to have a mixture of sons and daughters (males offspring can get killed by other wives and during succession disputes).

But not height ? In unarmed combat height is probably a substantial advantage (Though not to the extent modern heavyweight boxing suggests because steroids allow the bulking up of naturally gangling physiques).

Height is surely an advantage when fighting with clubs or stabbing spears so why are west Africans not very much taller than other people? I think it's because there was selection for avoiding projectile weapons in single combat or tribal warfare and the explosive speed required for leaping out of the path of a projectile is hindered by size beyond a certain point. Also, being big means presenting a bigger target.

So I guess nobody is going to suggest one of the more obvious causes for the economic blight in these African countries - European imperialism and exploitation of natural resources, anyone? Anyone? Africans aren't a monolithic group of helpless spearchuckers. (And siring several children isn't a trait unique to lower-class black men; this phenomenon appears in any ghetto, regardless of race or country.) These things really should go without saying.

Sex ratio at birth (males/females) is at its lowest in sub-Saharan populations and their diaspora. This might be an adaptation to high polygyny rates, i.e., a way of compensating for the high operational sex ratio.

Tod,

Sub-Saharan Africans are considerably larger in agricultural societies (high polygyny) than in hunter-gatherer societies (low polygyny). Body size has certainly increased from the latter to the former. And this increase has occurred over a time span of less than 10,000 years.

Anon,

In my post, I was criticizing the view (simplistic in my opinion) that a ban on polygyny would lead to immediate economic improvement in sub-Saharan Africa.

Your solution is to end European imperialism. Uh, where have you been for the past half-century?

Yes, there's such a thing as neo-imperialism. But those African countries that have maintained "neo-imperialist" ties have done better than those that haven't. Try comparing Botswana with Zimbabwe. Choose any measure you wish: life expectancy, GDP per capita, preservation of wild lands, economic equality, etc. Which country is better off?

You seem to like appealing to mainstream values. Well, choose your favorite value and tell me which country is better off. Hello?

In general, the poorest countries in Africa are the ones that have had the least experience with European imperialism.

"I mean if you look at the wikipedia page on national height, you'll see that many african countries are shorter than Japan. That doesn't seem right."

Masai are tall. Milk maybe?

Do Masai have a higher average IQ than most Africans?

I was just wondering because if IQ is somehow related to skull size and if the relevant aspect of average skull size varies with average height then if a group of people had a diet that made them taller on average then...

Sex ratio at birth (males/females) is at its lowest in sub-Saharan populations and their diaspora. This might be an adaptation to high polygyny rates, i.e., a way of compensating for the high operational sex ratio.

Yes, I would concur. Why give birth to males if they require a larger investment and they are likely to have a lower reproductive success.

Welcome to my blog! For the most part, this page will be an extension of my website, with comments relating to my research. But it will also branch out into more general discussions of human evolution.